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        <h3 id="一、flex-基本概念与常见属性"><a href="#一、flex-基本概念与常见属性" class="headerlink" title="一、flex 基本概念与常见属性"></a>一、<code>flex</code> 基本概念与常见属性</h3><p>&emsp;&emsp;刚接触 <code>flex</code>时，看到网上的各种视频或者文档教程，都略显冗长，因为一时半会儿压根不可能接受那么多东西，所以建议先将 <code>flex</code> 的概念做个大致了解，然后找案例跟着敲。</p>
<p><img src="flex.png" alt="flex"></p>
<p>&emsp;&emsp;我尽量说的通俗易懂，将一个标签 <code>display</code> 的值改为 <code>flex</code> 后，该容器就成了一个弹性盒子 <code>container</code> 。在 <code>container</code> 里面的子标签就成了 <code>item</code> (子项)。这个盒子默认情况下有两个轴，一个是 <code>cross axis</code> (主轴)，一个是 <code>cross axis</code> (一般称为交叉轴，为了方便这里叫它侧轴)。主轴默认是从左到右，侧轴从上到下。轴开始和结束被称为 <code>cross start</code> 和 <code>cross end</code> 所有子项默认情况下都是沿着主轴排列，单个子项所占据主轴的空间叫做 <code>main size</code> ，占据侧轴的空间叫做 <code>cross size</code> 。利用 <code>flex</code> 可以十分快捷并且灵活的布局。常见属性如下:</p>
<figure class="highlight css"><table><tr><td class="code"><pre><span class="line"><span class="comment">/* 父容器常用属性 */</span></span><br><span class="line"><span class="selector-tag">flex-direction</span>: <span class="selector-tag">row</span>; <span class="comment">/* 决定主轴方向，默认横向从左到右 */</span></span><br><span class="line"><span class="selector-tag">flex-wrap</span>: <span class="selector-tag">nowrap</span>; <span class="comment">/* 决定容器内子项是否换行，默认不换行 */</span></span><br><span class="line"><span class="selector-tag">flex-flow</span>: <span class="selector-tag">row</span> <span class="selector-tag">nowrap</span>; <span class="comment">/* 前两个属性的简写 */</span></span><br><span class="line"><span class="selector-tag">justify-content</span>: <span class="selector-tag">flex-start</span>; <span class="comment">/* 定义子项在主轴的对齐方式,默认为左对齐 */</span></span><br><span class="line"><span class="selector-tag">align-content</span>: <span class="selector-tag">stretch</span>; <span class="comment">/* 定义多根轴线的对齐方式，如果只有一根轴线，改属性无效 */</span></span><br><span class="line"><span class="selector-tag">align-items</span>: <span class="selector-tag">stretch</span>; <span class="comment">/* 定义子项在侧轴上的对齐方式，默认项目未设置高度时将占满整								  			个容器的高度 */</span></span><br></pre></td></tr></table></figure>

<figure class="highlight css"><table><tr><td class="code"><pre><span class="line"><span class="comment">/* 子项目常用属性 */</span></span><br><span class="line">    <span class="selector-tag">flex</span>: <span class="selector-tag">none</span>					 <span class="comment">/* 将容器分成几份，该项所占的份数 */</span></span><br><span class="line">    <span class="selector-tag">order</span>: 0;					 <span class="comment">/* 定义子项排列顺序,数值越小排列越靠前 */</span></span><br></pre></td></tr></table></figure>

<h4 id="注意事项："><a href="#注意事项：" class="headerlink" title="注意事项："></a>注意事项：</h4><ul>
<li><p>不控制子项的大小时，子项会根据父容器的大小撑满父容器。</p>
</li>
<li><p>父容器内只有在普通流的元素才会成为子项，并非所有元素都是子项。</p>
</li>
<li><p>使用 <code>flex</code> 后，<code>float</code>、<code>clear</code>以及 <code>vertical-align</code> 属性将会失效</p>
</li>
<li><p>可以将子项的 <code>display</code> 值改为 <code>flex</code> (嵌套)</p>
</li>
<li><p>如果元素标签的子级用的是 flex 布局，当前元素的上一级（也就是当前元素的父级）不能使用<code>positon: absolute/fixed</code></p>
</li>
<li><p><code>flex</code>在移动端兼容性非常好，PC 端最多只能兼容到 IE 9</p>
<p>以上内容，任何人第一次看都不可能有什么顿悟的感觉，毕竟理论是千篇一律的。鲁迅先生曾经说过:”碗酸辣汤，耳闻口讲的，总不如亲自呷一口的明白 。“</p>
</li>
</ul>
<h3 id="二、简单案例（只包含核心代码，通常需加上宽高颜色看效果）"><a href="#二、简单案例（只包含核心代码，通常需加上宽高颜色看效果）" class="headerlink" title="二、简单案例（只包含核心代码，通常需加上宽高颜色看效果）"></a>二、简单案例（只包含核心代码，通常需加上宽高颜色看效果）</h3><h6 id="emsp-emsp-1、水平垂直居中"><a href="#emsp-emsp-1、水平垂直居中" class="headerlink" title="&emsp;&emsp;1、水平垂直居中"></a>&emsp;&emsp;1、水平垂直居中</h6><figure class="highlight html"><table><tr><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">div</span> <span class="attr">class</span>=<span class="string">"container"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">div</span>&gt;</span><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight css"><table><tr><td class="code"><pre><span class="line"><span class="selector-class">.container</span> &#123;</span><br><span class="line">  <span class="attribute">display</span>: flex; <span class="comment">/*父容器改为弹性盒子flexbox*/</span></span><br><span class="line">  <span class="attribute">justify-content</span>: center; <span class="comment">/* 元素在主轴方向居中对齐 */</span></span><br><span class="line">  <span class="attribute">align-items</span>: center; <span class="comment">/* 元素在侧轴方向居中对齐 */</span></span><br><span class="line">&#125; <span class="comment">/* 只有一个子元素，所以里面的div就绝对居中container */</span></span><br></pre></td></tr></table></figure>

<h6 id="emsp-emsp-2、导航栏"><a href="#emsp-emsp-2、导航栏" class="headerlink" title="&emsp;&emsp;2、导航栏"></a>&emsp;&emsp;2、导航栏</h6><figure class="highlight html"><table><tr><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">ul</span> <span class="attr">class</span>=<span class="string">"container"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">li</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span>前端<span class="tag">&lt;/<span class="name">li</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">li</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span>DBA<span class="tag">&lt;/<span class="name">li</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">li</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span>大数据<span class="tag">&lt;/<span class="name">li</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">ul</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight css"><table><tr><td class="code"><pre><span class="line"><span class="selector-class">.container</span> &#123;</span><br><span class="line">  <span class="attribute">display</span>: flex; <span class="comment">/* flex容器内的元素默认不换行 */</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h6 id="emsp-emsp-3、flex-实现栅格布局"><a href="#emsp-emsp-3、flex-实现栅格布局" class="headerlink" title="&emsp;&emsp;3、flex 实现栅格布局"></a>&emsp;&emsp;3、<code>flex</code> 实现栅格布局</h6><figure class="highlight html"><table><tr><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">div</span> <span class="attr">class</span>=<span class="string">"container"</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">div</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br><span class="line">  <span class="comment">&lt;!-- 一个div代表一个栅格 --&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">div</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">div</span> <span class="attr">class</span>=<span class="string">"item"</span>&gt;</span><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">div</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight css"><table><tr><td class="code"><pre><span class="line"><span class="comment">/* 基础部分 */</span></span><br><span class="line"><span class="selector-class">.container</span> &#123;</span><br><span class="line">  <span class="attribute">display</span>: flex;</span><br><span class="line">&#125;</span><br><span class="line"><span class="selector-class">.item</span> &#123;</span><br><span class="line">  <span class="attribute">flex</span>: <span class="number">1</span>; <span class="comment">/* 容器内元素平均分配空间 */</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>&emsp;&emsp;上面的代码是平均分布不换行的，可以将父容器指定宽度后将 <code>flex-wrap</code> 属性更改为 <code>wrap</code> ，并且指定子项的宽度即可做成一个栅格。如下图。</p>
<p><img src="flex-grid.png" alt=""></p>
<h3 id="总结："><a href="#总结：" class="headerlink" title="总结："></a>总结：</h3><p>&emsp;&emsp;<code>flex</code>布局核心就是将元素 <code>display</code> 属性值改为 <code>flex</code> 后,通过调整主轴、侧轴的方向以及子项的位置和排列方式等来布局。掌握了常用属性后 <code>flex</code> 会改变你对布局的看法，其实掌握 <code>flex</code> 费时间的主要在于记属性。硬生生的记十几个属性实在是枯燥，所以这里推荐一个网站——<a href="https://flexboxfroggy.com/#zh-cn" target="_blank" rel="noopener">FLEXBOX FROGGY</a>通过游戏的方式可以很快的记住。通关后可以用 <code>flex</code> 仿做个移动端页面( <code>flex</code> 在移动端非常流行)，我仿做过一个携程页面）可以参考下（<a href="https://github.com/huiboxes/ctrip.m" target="_blank" rel="noopener">点这儿去</a>）。</p>

      
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